Microphone



Patented June 1930 Z i UNITED. PATENT OFFICE Emmy- JosErH ROUND, or LONDON,

ENGLAND, AssIenoR ro RADIO conroaA'rron or MEaIcA, A conronArron OF. DELAWARE mcaornonn A lication filed Jimem, 1927, Serial No. 200,339, and in Great Britain July 8, 1928.

I This invention relates to microphones, and

more particularly to microphones of the typeknown generally as carbon microphones, in which achange of current, consequent upon 5 a change of resistance. of carbon, is affected by the incidence of sound waves.

According to th1s lnventlon a carbon mlcrophone comprises a layer of carbonpowderor the like in contact with a' plurality of solid carbon electrodes, for example, carbon disks or rods, arranged transversely (i. e. the positive electrodes andthe negative electrodes lie more or less in a line at right'angles to the lines of incident sound), the whole arrangement being suchthat, in the main, change of current is produced in. the same direction as the pressure due to the sound waves. I Theinvention is illustrated in the accompanying drawings, in which Figure 1 is an end elevation partly in section and Figure 2 is a plan view showing the casing 2 in section. Figure 3 is an explanatory diagram showing the main directions of sound pressures and current lines of flow.

Referring to the figures, 1 are electrodes formed of rods of carbon, let into a massive .and substantially non-resonant body 2 of wood or other insulating -material, or of metal having an insulated surface or cover. The rods preferably lie flush with the insulated surface of the body 2, as shown, and are covered with a layer of carbon powder 3 which is held in lace by means of a thin diaphragm 4 of mica or other relatively nonyielding material. Alternate rods are con- 7 nected tog? ther, onexset to the positive ter-- minal of t e microphone and the other to the negative.

In a modification the rods 1 ofcarbon are replaced by a plurality of disks whose flat surfaces lie substantially flush with the insulated surface of the body 2. Where disks are employed, a satisfactor arrangement is to provide an equal num er of positive and negative disksevenly arranged, for example, over the surface of as uare.

It will be seen from Figure 3, (in which the dotted lines represent lines of pressure and the dashed lines represent lines of current) that when sound waves strike a mlcrophone,

known microphones of the transversely disthis direction of sound waves.

I have found that good results are Obtained,

owder does not exthe width or diameif the layer of carbon ceed 2 mm. in depth an .posed electrode type, in which the current changes occur substantially at right'angles to ter' of the rods or disks does'not exceed 5 mm. Y

Having now particularly. described and ascertained the nature of my said invention and in what manner the same is to be per formed, I declare that what I claim is:

1. In an electro-acoustic device, a plurality of stationary electrodes positioned to form a substantially flat surface,

ing, powdered material in contactwith the flat surface formed by said electrodes, means forapplying pressure-to said material whereby 1ts conductivity is changed and leads con- I necting alternate electrodes in parallel.

2. "In an electrov-acoustic device, a plurality of stationary electrodes mounted in a common plane, a circuit connected to said electrodes, a conducting, pulverulent material covering said electrodes and substantially rigid means adapted to ,be actuated by a sound wave for applying pressure to said material,

said means and said electrodes'lying in substantially parallel planes.

3. Armicrophone comprising at least three statlonary electrodes having their surfaces a layer of conduct-- arranged in parallel relation, a non-yielding diaphragm spaced from said surfacesland a mass of granular, conducting material filling the spacebetween said electrodes and said diaphragm.

4:. A microphone comprising a. body having a plurality of grooves therein, at least three electrodes'seated in said grooves, a diaphragm mounted adjacent-said electrodes, a

anass of conducting material contacting said electrodes and said diaphragm and a lead connectin two of said electrodes in parallel.

5. An e ectro-acoustic device comprising a plurality of electrodes arranged in parallel relation, a diaphragm spaced from said elec-' tro'des and a mass of conducting material v filling the space between said electrodes and said dia hragm, the thickness of said mass being su stantially two-fifths the width of p v saidelectrodes'.

6. An. acoustic device comprising a' 'sta-' tionary base member provided with at least three grooves formed therein, electrodes con- 1 10 forming to the curvature of. said grooves seated therein and having flat tops arranged in a plane, amass of granular, conducting material covering said electrodes, and a non yielding diaphragm arranged in .contact 15 with said mass and adapted to varythe resistanee thereof in accordance with incident sound waves. I

HENR JOSEPH ROUND. 

